1. Field of the Invention
The present invention relates to the slow motion display of frames of time sequential scene information through the use of longitudinal recording techniques.
2. Description Relative to the Prior Art
In longitudinal recording, a magnetic tape is advanced in the lengthwise direction of the tape while information is recorded along a track(s) that extends parallel to the direction of tape advancement. In general, a longitudinal recorder is classified as either a "rotary head" or "stationary head" longitudinal recorder, depending on whether the magnetic head(s) moves during the recording and playback operations. In the case of a rotary head longitudinal recorder, it is common practice to use a pair (or more) of magnetic heads mounted on the periphery of a drum about which the tape is partially wrapped. During recording, the drum rotates and the signals to be recorded are switched among the heads so that each head records during a time in which it contacts the tape. In a stationary head longitudinal recorder, on the other hand, the magnetic head(s) remains fixed in position while the magnetic tape is advanced longitudinally past the head.
Ideally, a technique for providing slow motion should be capable of forward slow motion (frames displayed in chronological order) and reverse slow motion (frames displayed in reverse chronological order), as well as being able to provide stop motion display of scene information. Further, the amount by which a moving object in the scene appears to be slowed down, hereinafter referred to as the speed reduction, should be variable.
It is generally known in the art to produce slow motion using a rotary head longitudinal recorder. For example, U.S. Pat. No. 3,942,188 discloses such a system wherein slow motion results from an adjustment of playback tape speed and head rotation rate so that each frame of scene information is played back and displayed a plurality of times. U.S. Pat. No. 3,294,902 discloses a similar system wherein portions of each frame are repeated so as to produce a slow motion effect. Apart from the inherent complexity attendant to the use of rotary heads, both of these systems suffer from a serious disadvantage in that the useful lives of the magnetic heads and the magnetic tape are relatively short. This problem is particularly acute for stop motion, or slow motion at high speed reductions because the playback heads repeatedly scrape the same segment of the magnetic tape. The resulting abrasive forces quickly wear the heads past the point of usefulness, as well as cause severe physical damage to the tape surface.
U.S. Pat. No. 3,588,334 discloses a slow motion video recording system that uses a longitudinal recorder of the stationary head type. In accordance with that disclosure, scene information is scanned by a camera that is operated at a fast frame rate and displayed at a slower frame rate to produce a slow motion display. The required frame rate conversion is accomplished by recording information at a fast tape speed and playing it back at a slower tape speed. While avoiding certain problems associated with excessive head and tape wear, this system is severely limited in the amount of speed reduction that it can provide. This is because the maximum obtainable speed reduction is limited by the frame rate at which the scene can be scanned by the camera. And, even assuming the use of present state-of-the-art camera technology, camera scan rates of more than a few times the standard scan rate are exceedingly difficult to achieve without a large sacrifice in resolution. The result is that such a system is limited in operation to relatively low speed reductions. It is also desirable to have a stop motion and a reverse motion capability, neither of which is available with the system disclosed in U.S. Pat. No. 3,588,334.